Frank A. Dinenno

Local inhibition of nitric oxide and prostaglandins independently reduces forearm exercise hyperaemia in humans

We tested the hypothesis that inhibition of synthesis of either nitric oxide (NO) or vasodilating prostaglandins (PGs) would not alter exercise hyperaemia significantly, but combined inhibition would synergistically reduce the hyperaemia. Fourteen subjects performed 20 min of moderate rhythmic forearm exercise (10% maximal voluntary contraction). Forearm blood flow (FBF) was measured by Doppler ultrasound. Saline or study drugs were infused (2 ml min−1) into the forearm via a brachial artery catheter to locally inhibit synthesis of NO and PGs during steady state exercise (NG‐nitro‐L‐arginine methyl ester (L‐NAME), 25 mg over 5 min to inhibit NO synthase (NOS); and ketorolac, 3 mg over 5 min to inhibit cyclooxygenase (COX)). After achieving steady state exercise over 5 min (control), L‐NAME was infused for 5 min, followed by 2 min saline, then by a 5 min infusion of ketorolac, and finally by 3 min of saline (n= 7). Drug order was reversed in seven additional subjects, such that single inhibition of NOS or COX was followed by combined inhibition. FBF during exercise decreased to 83 ± 2% of control exercise (100%) with NOS inhibition, followed by a transient decrease to 68 ± 2% of control during COX inhibition. However, FBF returned to levels similar to those achieved during NOS inhibition within 2 min (80 ± 3% of control) and remained stable through the final 3 min of exercise. When COX inhibition was performed first, FBF decreased transiently to 88 ± 4% of control (P < 0.01), and returned to control saline levels by the end of ketorolac infusion. Addition of L‐NAME reduced FBF to 83 ± 3% of control, and it remained stable through to the end of exercise. Regardless of drug order, FBF was ∼80% of steady state control exercise (P < 0.01) during the last 30 s of exercise. We conclude that (1) NO provides a significant, consistent contribution to hyperaemia, (2) PGs contribute modestly and transiently, suggesting a redundant signal compensates for the loss of vasodilating PGs, and (3) NO and PG signals appear to contribute independently to forearm exercise hyperaemia.

Endothelium-dependent vasodilatation and exercise hyperaemia in ageing humans: impact of acute ascorbic acid administration

Age‐related increases in oxidative stress impair endothelium‐dependent vasodilatation in humans, leading to the speculation that endothelial dysfunction contributes to impaired muscle blood flow and vascular control during exercise in older adults. We directly tested this hypothesis in 14 young (22 ± 1 years) and 14 healthy older men and women (65 ± 2 years). We measured forearm blood flow (FBF; Doppler ultrasound) and calculated vascular conductance (FVC) responses to single muscle contractions at 10, 20 and 40% maximum voluntary contraction (MVC) before and during ascorbic acid (AA) infusion, and we also determined the effects of AA on muscle blood flow during mild (10% MVC) continuous rhythmic handgrip exercise. For single contractions, the peak rapid hyperaemic responses to all contraction intensities were impaired ∼45% in the older adults (all P < 0.05), and AA infusion did not impact the responses in either age group. For the rhythmic exercise trial, FBF (∼28%) and FVC (∼31%) were lower (P= 0.06 and 0.05) in older versus young adults after 5 min of steady‐state exercise with saline. Subsequently, AA was infused via brachial artery catheter for 10 min during continued exercise. AA administration did not significantly influence FBF or FVC in young adults (1–3%; P= 0.24–0.59), whereas FBF increased 34 ± 7% in older adults at end‐exercise, and this was due to an increase in FVC (32 ± 7%; both P < 0.05). This increase in FBF and FVC during exercise in older adults was associated with improvements in vasodilator responses to acetylcholine (ACh; endothelium dependent) but not sodium nitroprusside (SNP; endothelium independent). AA had no effect on ACh or SNP responses in the young. We conclude that acute AA administration does not impact the observed age‐related impairment in the rapid hyperaemic response to brief muscle contractions in humans; however, it does significantly increase muscle blood flow during continuous dynamic exercise in older adults, and this is probably due (in part) to an improvement in endothelium‐dependent vasodilatation.

Impaired modulation of sympathetic α‐adrenergic vasoconstriction in contracting forearm muscle of ageing men

Recent evidence indicates that older healthy humans demonstrate greater vasoconstrictor tone in their active muscles during exercise compared with young adults. Therefore, we tested the hypothesis that the normal ability of muscle contractions to blunt sympathetic α‐adrenergic vasoconstriction (functional sympatholysis) is impaired with age in healthy humans. We measured forearm blood flow (FBF; Doppler ultrasound) and calculated the forearm vascular conductance (FVC) responses to α‐adrenergic receptor stimulation during rhythmic handgrip exercise (15% maximum voluntary contraction) and during a control non‐exercise vasodilator condition (intra‐arterial adenosine infusion) in seven young (25 ± 2 years) and eight healthy older men (65 ± 2 year). FVC responses to intra‐arterial tyramine (evokes endogenous noradrenaline release), phenylephrine (α1‐agonist) and clonidine (α2‐agonist) were assessed. In young men, the vasoconstrictor responses to tyramine (−25 ± 1 versus−56 ± 6%), phenylephrine (−11 ± 4 versus−39 ± 4%) and clonidine (−12 ± 4 versus−38 ± 5%; all P < 0.005) were blunted during exercise compared with adenosine. In contrast, exercise did not significantly blunt the response to tyramine (−30 ± 2 versus−36 ± 7%; P= 0.4) or phenylephrine (−16 ± 2 versus−19 ± 3%; P= 0.3) in older men, but did attenuate the response to clonidine (−22 ± 3 versus−37 ± 6%; P < 0.05). The magnitude of functional sympatholysis, calculated as the difference in the vasoconstrictor responses during adenosine infusion and exercise, was significantly lower in older compared with young men in the presence of tyramine (−6 ± 7 versus−31 ± 6%), phenylephrine (−3 ± 3 versus−28 ± 4%) and clonidine (−15 ± 4 versus−26 ± 3%; all P < 0.05). We conclude that ageing is associated with impaired functional sympatholysis in the vascular beds of contracting forearm muscle in healthy men. These findings might help explain the greater skeletal muscle vasoconstrictor tone and reduced blood flow during large muscle dynamic exercise in older adults.

Mechanical influences on skeletal muscle vascular tone in humans: insight into contraction-induced rapid vasodilatation

We tested the hypothesis that mechanical deformation of forearm blood vessels via acute increases in extravascular pressure elicits rapid vasodilatation in humans. In healthy adults, we measured forearm blood flow (Doppler ultrasound) and calculated forearm vascular conductance (FVC) responses to whole forearm compressions and isometric muscle contractions with the arm above heart level. We used several experimental protocols to gain insight into how mechanical factors contribute to contraction‐induced rapid vasodilatation. The findings from the present study clearly indicate that acute increases in extravascular pressure (200 mmHg for 2 s) elicit a significant rapid vasodilatation in the human forearm (peak ΔFVC∼155%). Brief, 6 s sustained compressions evoked the greatest vasodilatation (ΔFVC∼260%), whereas the responses to single (2 s) and repeated compressions (five repeated 2 s compressions) were not significantly different (ΔFVC∼155%versus∼115%, respectively). This mechanically induced vasodilatation peaks within 1–2 cardiac cycles, and thus is dissociated from the temporal pattern normally observed in response to brief muscle contractions (∼4–7 cardiac cycles). A non‐linear relation was found between graded increases in extravascular pressure and both the immediate and peak rapid vasodilatory response, such that the responses increased sharply from 25 to 100 mmHg, with no significant further dilatation until 300 mmHg (maximal ΔFVC∼185%). This was in contrast to the linear intensity‐dependent relation observed with muscle contractions. Our collective findings indicate that mechanical influences contribute largely to the immediate vasodilatation (first cardiac cycle) observed in response to a brief, single contraction. However, it is clear that there are additional mechanisms related to muscle activation that continue to cause and sustain vasodilatation for several more cardiac cycles after contraction. Additionally, the potential contribution of mechanical influences to the total contraction‐induced hyperaemia appears greatest for low to moderate intensity single muscle contractions, and this contribution becomes less significant for sustained and repeated contractions. Nevertheless, this mechanically induced vasodilatation could serve as a feedforward mechanism to increase muscle blood flow at the onset of exercise, as well as in response to changes in contraction intensity, prior to alterations in local vasodilating substances that influence vascular tone.

Graded sympatholytic effect of exogenous ATP on postjunctional α-adrenergic vasoconstriction in the human forearm: implications for vascular control in contracting muscle

Recent evidence suggests that adenosine triphosphate (ATP) can inhibit vasoconstrictor responses to endogenous noradrenaline release via tyramine in the skeletal muscle circulation, similar to what is observed in contracting muscle. Whether this involves direct modulation of postjunctional α‐adrenoceptor responsiveness, or is selective for α1‐ or α2‐receptors remains unclear. Therefore, in Protocol 1, we tested the hypothesis that exogenous ATP can blunt direct postjunctional α‐adrenergic vasoconstriction in humans. We measured forearm blood flow (FBF; Doppler ultrasound) and calculated the vascular conductance (FVC) responses to local intra‐arterial infusions of phenylephrine (α1‐agonist) and dexmedetomidine (α2‐agonist) during moderate rhythmic handgrip exercise (15% maximum voluntary contraction), during a control non‐exercise vasodilator condition (adenosine), and during ATP infusion in eight young adults. Forearm hyperaemia was matched across all conditions. Forearm vasoconstrictor responses to direct α1‐receptor stimulation were blunted during exercise versus adenosine (ΔFVC =−11 ± 3%versus−39 ± 5%; P< 0.05), and were abolished during ATP infusion (−3 ± 2%). Similarly, vasoconstrictor responses to α2‐receptor stimulation were blunted during exercise versus adenosine (−13 ± 4%versus−40 ± 8%; P< 0.05), and were abolished during ATP infusion (−4 ± 4%). In Prototol 2 (n= 10), we tested the hypothesis that graded increases in ATP would reduce α1‐mediated vasoconstriction in a dose‐dependent manner compared with vasodilatation evoked via adenosine. Forearm vasoconstrictor responses during low dose adenosine (−38 ± 3%) and ATP (−33 ± 2%) were not significantly different from rest (−40 ± 3%; P> 0.05). In contrast, vasoconstrictor responses during moderate (−22 ± 6%) and high dose ATP (−8 ± 5%) were significantly blunted compared with rest, whereas the responses during adenosine became progressively greater (moderate =−48 ± 4%, P= 0.10; high =−53 ± 6%, P< 0.05). We conclude that exogenous ATP is capable of blunting direct postjunctional α‐adrenergic vasoconstriction, that this involves both α1‐ and α2‐receptor subtypes, and that this is graded with ATP concentrations. Collectively, these data are consistent with the conceptual framework regarding how muscle blood flow and vascular tone are regulated in contracting muscles of humans.

Ageing and leg postjunctional α-adrenergic vasoconstrictor responsiveness in healthy men

Muscle sympathetic vasoconstrictor nerve activity increases with advancing age, but does not result in elevated forearm vasoconstrictor tone because of a selective reduction in α1‐adrenoceptor responsiveness. In contrast, the leg circulation of older adults is under greater tonic sympathetic vasoconstriction, but it is unclear whether α‐adrenoceptor responsiveness is altered with age. In the present study, we tested the hypothesis that postjunctional α‐adrenergic vasoconstrictor responsiveness is reduced in the leg circulation with age. We measured femoral blood flow (Doppler ultrasound) and calculated the femoral vascular conductance (FVC) responses to α‐adrenoceptor stimulation during local blockade of β‐adrenoceptors in 12 young (24 ± 1 year) and seven healthy older men (62 ± 2 year). Whole‐leg vasoconstrictor responses to local intrafemoral artery infusions of tyramine (evokes noradrenaline (NA) release), phenylephrine (α1‐agonist) and dexmedetomidine (α2‐agonist) were assessed. Consistent with previous data, resting femoral blood flow and FVC were ∼30% lower in older compared with young men (P < 0.05). Maximal vasoconstrictor responses to tyramine (−30 ± 3 versus−41 ± 3%), phenylephrine (−25 ± 4 versus−45 ± 5%), and dexmedetomidine (−22 ± 4 versus−44 ± 3%) were all significantly lower in older compared with young men (all P < 0.05). Our results indicate that human ageing is associated with a reduction in leg postjunctional α‐adrenoceptor responsiveness to endogenous NA release, and this reduction is evident for both α1‐ and α2‐adrenoceptors. However, given that basal leg vascular conductance is reduced with age and is primarily mediated by sympathetic vasoconstriction, impaired α‐adrenoceptor responsiveness does not negate the ability of the sympathetic nervous system to evoke greater tonic vasoconstriction in the leg vasculature of older men.

α‐Adrenergic Control of Skeletal Muscle Circulation at Rest and During Exercise in Aging Humans

Aging is associated with many changes in autonomic nervous system function that often lead to impairments in the normal ability to respond to physiological stressors commonly encountered in daily life. In addition, many of these chronic age‐related changes in autonomic‐circulatory function can potentially predispose the older adult to elevated risk for acute and chronic cardiovascular complications. One of the most pronounced and repeatable findings with respect to changes in the autonomic nervous system with human aging is the progressive increase in basal muscle sympathetic nerve activity (MSNA) directed to skeletal muscle vascular beds. Although the mechanism(s) underlying this sustained age‐associated increase in MSNA are not completely understood, several changes in sympathetic α‐adrenergic function occur with age. In this review, the authors discuss how aging affects ( 1 ) α‐adrenergic control of skeletal muscle vascular tone under resting conditions and the differences that exist in this control of the upper and lower limbs (forearm vs leg circulation); ( 2 ) vasoconstrictor responsiveness to endogenous norepinephrine release, as well as the specific responsiveness of postjunctional α1‐ and α2‐adrenergic receptors; and ( 3 ) sympathetic α‐adrenergic control of muscle blood flow and vascular tone during exercise in humans. Further, they discuss how these changes in sympathetic α‐adrenergic control of skeletal muscle blood vessels have important physiological and clinical implications for the aging human.

Augmented skeletal muscle hyperaemia during hypoxic exercise in humans is blunted by combined inhibition of nitric oxide and vasodilating prostaglandins

Non‐technical summaryu2002 Blood flow to muscle increases during exercise in order to deliver more oxygen. When there is less oxygen in the blood, as in systemic hypoxia, blood flow also increases. If exercise occurs during hypoxia, the blood flow response is greater than during normal oxygen conditions, but the mechanisms by which this happens are not clear. We show that two substances that the body produces, nitric oxide and prostaglandins, contribute to this increased blood flow during hypoxic exercise. These results help us better understand how oxygen delivery is regulated and may be especially important for populations which are unable to produce these substances that help increase blood flow.

Combined inhibition of nitric oxide and vasodilating prostaglandins abolishes forearm vasodilatation to systemic hypoxia in healthy humans

Non‐technical summaryu2002 During hypoxia, there is less oxygen in the air we breathe and also in the blood being pumped away from the heart. Our blood vessels must relax in order to deliver more blood to match the resting oxygen demand of the muscles. The way in which multiple systems in the body coordinate this response is not well known. We examined the local response of the blood vessels to a hypoxic stimulus and show that two substances that the body produces, nitric oxide and prostaglandins, are necessary to cause relaxation of the blood vessels and increases in blood flow. These results help us better understand how oxygen delivery is regulated and may be especially important for populations that are unable to produce these substances that help increase blood flow, such as people with sleep apnoea, heart failure and diabetes.

Mechanisms of rapid vasodilation after a brief contraction in human skeletal muscle

A monophasic increase in skeletal muscle blood flow is observed after a brief single forearm contraction in humans, yet the underlying vascular signaling pathways remain largely undetermined. Evidence from experimental animals indicates an obligatory role of vasodilation via K⁺-mediated smooth muscle hyperpolarization, and human data suggest little to no independent role for nitric oxide (NO) or vasodilating prostaglandins (PGs). We tested the hypothesis that K⁺-mediated vascular hyperpolarization underlies the rapid vasodilation in humans and that combined inhibition of NO and PGs would have a minimal effect on this response. We measured forearm blood flow (Doppler ultrasound) and calculated vascular conductance 10 s before and for 30 s after a single 1-s dynamic forearm contraction at 10%, 20%, and 40% maximum voluntary contraction in 16 young adults. To inhibit K⁺-mediated vasodilation, BaCl₂ and ouabain were infused intra-arterially to inhibit inwardly rectifying K⁺ channels and Na⁺-K⁺-ATPase, respectively. Combined enzymatic inhibition of NO and PG synthesis occurred via NG-monomethyl-L-arginine (L-NMMA; NO synthase) and ketorolac (cyclooxygenase), respectively. In protocol 1 (n = 8), BaCl₂ + ouabain reduced peak vasodilation (range: 30-45%, P < 0.05) and total postcontraction vasodilation (area under the curve, ~55-75% from control) at all intensities. Contrary to our hypothesis, L-NMMA + ketorolac had a further impact (peak: ~60% and area under the curve: ~80% from control). In protocol 2 (n = 8), the order of inhibitors was reversed, and the findings were remarkably similar. We conclude that K⁺-mediated hyperpolarization and NO and PGs, in combination, significantly contribute to contraction-induced rapid vasodilation and that inhibition of these signaling pathways nearly abolishes this phenomenon in humans.